An Invisible War – Overview on the role of UV-C in fighting COVID-19 By Biswajit Sengupta - June 29, 2020

by Biswajit Sengupta

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UV-C vs COVID 19

The invisible pathogen COVID 19 has suddenly taken our entire planet by surprise at the beginning of 2020. The invasion was so sudden that even the World Health Organization (WHO) took time to declare this as a pandemic. At a time when the entire world is trying to arrive at plausible solutions to arrest the spread of this rampaging virus, our Lighting fraternity has joined the fight by concentrating more on studies of non-visual impacts of light. An invisible part of the optical radiation spectrum, ultraviolet radiation has more energy than its visible counterpart due to its shorter wavelengths. The ultraviolet spectrum comprises UV-A (400 nm to 315 nm); UV-B (315 nm to 280 nm); UV-C (280 nm to 100 nm) – classification as defined by CIE. Whenever ultraviolet radiation is used for germicidal purposes, it is known as GUV (Germicidal Ultraviolet ) radiation. It has been found to be highly effective in deactivating or killing viruses, bacteria, fungi, and protozoa. Due to complex photochemistry of UV- C component of sunrays with oxygen molecule at stratospheric level (about 16 km to 32 km above the earth surface) formation of ozone layer takes place, which in turn blocks irradiance of the actinic component of UV on human occupants. If nature had failed to stop the actinic radiance, it would have lead to physical hazards caused by photochemical and thermal reactions on human skin, cornea, and retina. So while trying to artificially generate UV-C, scientists have to monitor the wavelengths, angular subtense, exposure time, and finally physiological sensitivity to various wavelengths. The radiant exposure which determines the dosage is dependent on the relative humidity and the kind of infectious agent it is required to tackle. Earlier studies have demonstrated the effectiveness of UV-C at 254 nm on surfaces contaminated with the Ebola virus and also during the influenza outbreak in Jordan in 1961.UV-C has proved its usefulness in the treatment of water disinfection and also air (CIE 2003) in Air Handling Units for quite some time now. Upper-air disinfection of circulating air by UV-C has been successful in limiting the spread of tuberculosis and has the recommendation of WHO. Though serious research is underway to arrest the spread of SARS-CoV-2 with the application of UV-C for surface disinfection, its effectivity is yet to be published and proved. The major difficulty in containing the spread of SARS-CoV-2 arises from the observation recorded on the duration of its stay on fomites, which again vary on environmental factors and surface material. The successful disinfection of touch surfaces, therefore, depends on the determination of the location of the UV equipment and also proper reduction of shadow effects. Shadow effects play a deterrent role, for example, on frequently touched surfaces like door handles, latches, etc.

It has, however, been proved time and again that in actual practice UV-C radiation can deactivate pathogens by damaging their DNA. Shorter the wavelengths, more effective are the damage to pathogens. But due to complicated oxygen-ozone photochemistry, it has been observed that a significant increase in ozone production occurs at a wavelength of 200 nm or lower. That UV-C, at wavelength produced by the mercury resonance line of 253.7 nm, possesses the ability to kill bacteria was observed by researchers way back in 1877. Protein, particularly DNA was found to strongly absorb this wavelength. As UV radiation is found to trigger photochemical and thermal reactions on human skin it is always safe to keep the area being sterilized free from occupants. CIE set a threshold limit of allowable daily exposure dose to 60 J-sqm for 8-hour continuous exposure to UV-C radiation at 254 nm. Hazards associated with UV-C overexposures are transient corneal irritation(photokeratitis); conjunctival irritation (photo conjunctivitis) and skin irritation (erythema). These effects disappear within a 24-48 hour period, as per findings in CIE 187:2010 Photo carcinogenesis Risks from Germicidal Lamps. It is also mentioned in the report that the penetration in human skin is superficial only and, therefore, does not produce lasting biological damage. Since UV-C has the ability to deactivate any microbe, virus, fungus, or spore a proper dose has to be determined. The parameters which have to be considered for working out the required dose are UV-C wavelength, type of pathogen, the environment around pathogen, aerosol droplet size (with the virus inside).

GUV system

a) Germicidal Lamps

Light sources that radiate in the UV-C spectrum are known as Germicidal Lamps. UV-C radiation gets absorbed by the DNA & RNA microorganisms. This changes its structure and halts replication.

Low-Pressure Mercury Vapour Lamp – in its simplest form, a fluorescent tube without phosphors, produces monochromatic emission at 253.7 nm suitable for the germicidal application. Low-pressure mercury lamp emitting at 185 nm is also available and is suitable for the oxidation of surfaces. These lamps have been used for the disinfection of air, water, and surface for over forty years. The lamps have high efficiency, almost 40% UV-C generation. Although this type of lamp contains mercury and has a lifetime of 10000 burning hours, its survival in the market is mainly due to its low cost. Its spectral irradiance peaks to 1750 mW/sqm at 253.7 nm.

b) Xenon Lamp

This lamp also contains mercury and its lifetime is lower than 10000 hours. The lamp operates on pulsed mode and produces a lot of visible light. Covers the entire UV spectrum, and peaks around 230 nm. It is also costlier than Low-Pressure Mercury Lamp.

c) Excimer Lamp

Kr-Cl excimer emits a narrow band at 222 nm. Regarded as a better germicidal lamp because of its higher energy per photon. Normally it is recommended for use when people are not around.

d) Solid-State Light (SSL)

Crystal Growth technology has led to the development of a deep ultraviolet LED. At current wavelengths for disinfection, these solid-state sources have already proved to be more efficient than their conventional predecessors. They are being used in high power, high irradiance applications. Fabrication of deep-ultraviolet (UV-C) LEDs involves depositing a film of the semiconductor alloy aluminum gallium nitride (AlGaN) on a substrate of a sapphire substrate. Research is underway which suggests the use of silicon carbide (SiC) as a substrate. This substitute they suggest will bring down the cost of UV-C LED and at the same time increase the efficacy. Optical intensity has been found to be high at 275 nm. The best part of the UV-C LED source is its size, thus allowing system design flexibility. A portable handheld product can be made available to access, control microbes, and prevent infection on portable healthcare equipment and smaller touchpoints. This will make possible point-of-care deployment, particularly sterilization of frequently touched surfaces such as personal electronics, medical trolleys, and diagnostic devices.


The parameters which are considered for designing and developing efficient luminaires for the UV spectrum are transmission and reflectance. As UV is reactive on standard transmitting materials, the selection of the right material plays a major role in achieving over 90% transmittance without significant aging and degradation over a period of time. Synthetic fused silica provides good transmittance from 180 nm and above. Other UV transmitting materials for various wavelengths in the UV spectrum are. Zinc selenide (ZnSe), calcium fluoride (CaF2), lithium fluoride (LiF) and magnesium fluoride (MgF2).Another very important consideration for selecting the above materials is that these glasses have very low fluorescence properties. UV transmitting Acrylics are also available for use in place of glass.

For reflector anodized ultra-high purity Aluminium has been the best choice because of its commercial availability. But it has been observed that the metal does not produce a uniform Lambertian distribution of light. Uniform diffused reflection of UV takes place with sintered microporous PTFE (polytetrafluoroethylene) reflector, and this is the best material choice for a range of UV reflectivity. A good sintered PTFE reflector can provide over 97% average reflectance from 220 nm to 400 nm. With its ability to scatter light in all directions, it ensures the spread of UV light evenly over a surface. This uniform distribution, in turn, ensures the elimination of 'cold spots' where a virus can survive. A PTFE reflector maintains its efficacy up to a working temperature of 260 degrees C. Many common polymer materials, such as polyethylene, polycarbonate, and nylon age and degrade under the influence of UVC.

Testing & Measurement

Germicidal UV is a powerful tool in fighting COVID 19 because its irradiation for inactivating viruses in the air, water, and solid surfaces is a proven methodology. If not used properly it can pose risks to human health. With the outbreak of the novel coronavirus, there are products available for household applications. With long lockdowns in force, people go for online shopping. Most of the products have been found to have dubious safety features and inadequate safety instructions. The germicidal effectiveness of UV-C radiation depends on its dose in micro Joule per square cm and exposure duration. Equipment is in place for accurately testing UV systems, such as high precision spectroradiometers with very low stray light and PTFE coated integrating sphere. Even an automated solution is in place with full software guidance, calculation, and classification in compliance with IEC 62471, a globally accepted comprehensive standard that considers skin, cornea, and retinal hazards. Radiometers have been developed for the measurement of UV-C irradiance over a very wide dynamic range to beyond 100 mW/sqcm with a resolution of 0.0001 microwatts/sqcm (spectral responsivity is calibrated from 200 nm to 300 nm).

The above images are courtesy webinar of Robert Karlicek, Jr., director of the Lighting Enabled Systems and Applications (LESA) Center June 18, 2020. The webcast topic was "Germicidal UV-C radiation: Fact and Fiction about Killing Pathogens."


CIE 187:2010 Photo carcinogenesis Risks from Germicidal Lamps

CIE Issues Position Statement on the Use of UV to Manage COVID-19 Transmission Risk

Risks and Rewards of Germicidal UV by Craig DiLouie

Lighting Europe Calls for UV-C Products to be Allowed on EU Market

By Craig DiLouie

Burhan K. SaifAddin, Abdullah S. Almogbel, Christian J. Zollner, Feng Wu, Bastien Bonef, Michael Iza, Shuji Nakamura, Steven P. DenBaars, James S. Speck: AlGaN deep-ultraviolet Light-Emitting Diodes Grown on SiC Substrates. ACS Photonics, 2020

UV-C LUMINAIRES Specialized Disinfection Lighting --- LUXIONA POLAND S.A. Catalogue

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